U.S. patent number 4,296,277 [Application Number 06/079,186] was granted by the patent office on 1981-10-20 for electronic voice detector.
This patent grant is currently assigned to Feller AG. Invention is credited to Hansruedi Daneffel.
United States Patent |
4,296,277 |
Daneffel |
October 20, 1981 |
Electronic voice detector
Abstract
A voice detector for supplying a control signal in an automatic
telephone answering apparatus in response to a voice signal
received from a telephone line. A first one of two parallel signal
channels generates a first digital signal for an incoming signal
having a minimum amplitude with a minimum duration. The second
signal channel generates a second digital signal for an incoming
signal having a minimum number of consecutive cycles with periods
within a predetermined range corresponding to the frequency range
of a ringing signal. A logic element combines the first and second
digital signals to produce the control signal if simultaneously the
first digital signal is present and the second digital signal is
absent. Thereby, the control signal is produced only in response to
an incoming voice signal, but not in response to a ringing or a
noise signal.
Inventors: |
Daneffel; Hansruedi (Uerikon,
CH) |
Assignee: |
Feller AG (Horgen,
CH)
|
Family
ID: |
4358612 |
Appl.
No.: |
06/079,186 |
Filed: |
September 26, 1979 |
Foreign Application Priority Data
|
|
|
|
|
Sep 26, 1978 [CH] |
|
|
10033/78 |
|
Current U.S.
Class: |
704/246; 379/80;
381/101; 704/E11.003 |
Current CPC
Class: |
H04M
1/654 (20130101); G10L 25/78 (20130101) |
Current International
Class: |
G10L
11/00 (20060101); G10L 11/02 (20060101); H04M
1/65 (20060101); H04M 1/654 (20060101); H04M
001/64 () |
Field of
Search: |
;179/1VC,1VL,1HF,6R,1.1VC,6.14,81B ;340/148 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Popek; Joseph A.
Attorney, Agent or Firm: Weiser, Stapler & Spivak
Claims
I claim:
1. An electronic voice detector for use in an automatic telephone
answering apparatus, said voice detector having input means for
receiving audio signals from a telephone line, and output means for
supplying a control signal indicating the presence of an incoming
voice signal, wherein between said input means and said output
means a first signal channel and a second signal channel are
disposed in parallel configuration, said signal channels being
commonly connected to said input means and said signal channels
being connected to said output means through a logic means, said
first signal channel comprising signal threshold means for
generating an output signal if the signal from the input means has
an amplitude exceeding a predetermined level, and signal delay
means connected to the output of said threshold means for
generating an output signal if said output signal from said
threshold means has a duration exceeding a predetermined time
interval, said second signal channel comprising period measuring
means for measuring the period of each cycle of the signal from
said input means and for generating a first output signal at a
first output if the period of the cycle is within a predetermined
range, and a second output signal at a second output if the period
of the cycle is outside said predetermined range, and counting
means having a counting input, a reset input and an output, said
counting and reset inputs being connected to said first and second
outputs, respectively, of said period measuring means, said
counting means generating an output signal for a predetermined
number of consecutive output signals from said first output of said
period measuring means, and being reset by an output signal from
said second output of said period measuring means, said output of
said delay means being connected to a first input of said logic
means and said output of said counting means being connected to a
second input of said logic means, said logic means being adapted to
supply the control signal to said output means with the presence of
an output signal from said delay means and with the simultaneous
absence of an output signal from said counting means.
2. The voice detector of claim 1, wherein said input means include
a bandpass filter common to the first and the second signal
channels.
3. The voice detector of claim 2, wherein the bandpass filter has a
pass band approximately extending from 300 Hz to 3600 Hz.
4. The voice detector of claim 1, wherein a signal rectifying means
is disposed between said threshold means and said delay means in
said first signal channel.
5. The voice detector of claim 1, wherein said delay means in said
first signal channel comprises means for responding to input
signals having a duration of at least approximately 3 milliseconds
to produce a digital output signal having a duration of at least
approximately 0.5 seconds.
6. The voice detector of claim 1, wherein between said input means
and said period measuring means in said second signal channel a
symmetrical limiter is connected.
7. The voice detector of claim 1, wherein said counting means
includes a comparator for continuously comparing the count in said
counting means with a predetermined value and for supplying an
output signal when the count reaches said predetermined value.
8. The voice detector of claim 1, wherein said logic means includes
an AND gate having two inputs, a first input of said AND gate being
connected to the output of said delay means and a second input of
said AND gate being connected, through an inverter, to the output
of said counting means.
9. The voice detector of claim 8, wherein a signal rectifying means
is disposed between said threshold means and said delay means in
said first channel, and wherein said threshold means, said
rectifying means and said delay means in combination include an
operational amplifier having a reference input, a signal input and
an output, for holding each of said inputs at a different fixed
potential, a transistor switch operable by the signal from the
output of said operational amplifier, a first time delay circuit
including in series said transistor switch, a first resistor and a
capacitor, a second time delay circuit including in series a second
resistor arranged as a load resistor of said transistor switch,
said first resistor and said capacitor, said second resistor having
a substantially higher resistance value than said first resistor,
whereby with said transistor switch being conducting said capacitor
is discharged through said first resistor and said tansistor switch
at a fast rate, and with said transistor switch being
non-conducting said capacitor is charged through said second and
first resistors at a slow rate, said first input of said AND gate
being connected to said capacitor.
10. The voice detector of claim 9, wherein a second inverter is
disposed between said capacitor and said first input of said AND
gate.
Description
The invention relates to a voice detector for use in an automatic
telephone answering apparatus in which the voice detector supplies
a control signal in response to a voice signal from a telephone
line. More particularly, it relates to a voice detector having
input means for receiving audio signals from the telephone line,
and output means for supplying the control signal indicating the
presence of an incoming voice signal.
Voice detectors of that type, which sometimes are also designated
as speech switches, are known, for example, from the Canadian
Patent Specification No. 961,186 and from the corresponding British
Patent Specification No. 1,388,037 as well as from the U.S. Pat.
No. 3,937,893 by Willy Muller et al. Such voice detectors have the
task of detecting a voice signal arriving over the telephone line,
discriminating the voice signal against any other signal in the
audio frequency range such as a ringing signal from the exchange or
noise signals which may arrive over the telephone line, and
supplying a voice signal responsive control signal which is used in
the telephone answering apparatus for performing switching
functions disclosed in detail in the aforementioned patent
specifications.
Voice detectors of the present type show the inherent problem of
reliably recognizing a voice signal in the voice frequency range of
approximately 600 Hz to 3600 Hz, but to completely suppress any
ringing signal which arrives over the telephone line with a high
amplitude and a frequency which is contiguous to the lower end of
the voice frequency range, and which normally is between about 400
and 500 Hz. The provision of a filter having steep response slopes
is practically useless, because in most cases the ringing signals
show a harmonic distortion attaining 30%. Therefore, higher
harmonics will be present with high amplitudes over the entire
voice frequency range. Other more sophisticated methods of
suppressing ringing signals or noise signals in the voice frequency
range, such as methods of frequency analyses, result in expensive
circuit realizations which, in addition are subject to frequent
troubles and failures.
Accordingly, it is the principal object of the present invention to
provide a voice detector of the initially described type which
makes possible an unambiguous and reliable detection of voice
signals with the aid of relatively simple and stable circuit
means.
It is a further object of the invention to provide a voice detector
including circuit means for determining the presence of a ringing
signal and for deriving therefrom a control signal representative
of a voice signal in the incoming audio signal.
These and other objects which will appear are achieved in
accordance with the invention by providing a voice detector in
which between the input means receiving audio signals from the
telephone line and the output means supplying the desired control
signal, two signal channels are arranged in a parallel
configuration. The first signal channel comprises threshold means
and delay means connected thereto. The threshold means generate an
output signal if the signal from the input means has an amplitude
exceeding a predetermined level. The delay means generate an output
signal if the output signal from the threshold means has a duration
exceeding a predetermined time interval. The second signal channel
comprises period measuring means and counting means connected
thereto. The period measuring means measure the period of each
cycle of the signal from the input means. If the period of a cycle
is within a predetermined range corresponding to the frequency
range of the ringing signal, a first signal from the period
measuring means is supplied to a counting input of the counting
means. Whenever the measured period of a cycle falls outside the
predetermined range, a second signal from the period measuring
means is supplied to a reset input of the counting means.
Consecutive counting signals from the period measuring means are
counted in the counting means upto a predetermined count, whereupon
the counting means generate an output signal. The output signals
from the delay means in the first signal channel and from the
counting means in the second signal channel are supplied to a logic
circuit means which supplies the control signal to the output means
of the voice detector provided the output signal from the delay
means is present, whilst the output signal from the counting means
simultaneously is absent.
The invention will be further described with reference to
illustrative embodiments and the accompanying drawings wherein
FIG. 1 is a block diagram of a voice detector according to the
invention; and
FIG. 2 is an individual circuit diagram of a portion of the voice
detector.
Referring to FIG. 1, the voice detector is designed to be
connected, in an automatic telephone answering apparatus, between a
telephone line terminal of the apparatus and a change-over device
of the apparatus which switches the apparatus either to the
operational state of recording an incoming telephone message when
the voice of the caller is detected, or to the operational state of
supplying a final announcement text to the telephone line when the
message is terminated and no voice is detected from the caller.
Connections of a voice detector in an automatic telephone answering
apparatus are illustrated, by way of example, in the patent
specifications referred to above.
An input terminal 1 of the voice detector receives a signal S1
indirectly from other circuits of the telephone answering apparatus
(not shown) which are in connection with the telephone line, such
as a line transformer, relay contacts, an amplifier. The signal S1
may include a ringing signal from the telephone exchange and/or
voice signals from the caller as well as noise signals which may be
present on the telephone line. The signal S1 is supplied to a
bandpass filter 2, the pass band of which extends over the useful
frequency range of the incoming signals including the ringing
signal, i.e., approximately from 300 Hz to 3600 Hz. The bandpass
filter may have a moderate to low Q-factor (circuit-Q) as it serves
the only purpose of blocking noise signals outside the useful
frequency range.
The output signal S2 of the bandpass filter 2 is simultaneously
furnished to a first circuit or signal channel 3 and to a second
circuit or signal channel 4. Generally, the signal channel 3
transforms the input signal S2 into a first digital signal provided
its amplitude is higher than a predetermined minimum value and its
duration is longer than a predetermined value, and the signal
channel 4 provides a second digital signal in response to a
predetermined period of the input signal S2 and in response to a
predetermined number of successive occurences of such periods.
The first signal channel 3 comprises, in successive connections, a
threshold switch 5 supplying an output signal S3, a rectifier
circuit 6 supplying an output signal S4, and a delay switch 7
supplying an output signal S5. The threshold switch changes its
switch state and supplies the output signal S3 when the input
signal S2 reaches or exceeds a predetermined amplitude. The delay
switch 7 suppresses those rectified output signals S4 from the
rectifier circuit 6 which have a shorter duration than a
predetermined value of, for example, 3 milliseconds. By that
feature, short noise pulses and peaks are effectively suppressed.
The delay switch 7 is further designed to stretch an output signal
S4 from the rectifier circuit 6, which is of longer duration than
said minimum value, to a digital output signal S5 having a
predetermined minimum duration of, for example, 0.5 seconds. Thus,
the digital output signal S5 from the delay switch 7 occurs
provided the signal S1 arriving at the input terminal 1 from the
telephone line lies within the pass band of the bandpass filter 2,
has a minimum amplitude, and has a minimum duration.
The second signal channel 4 serves the purpose of detecting whether
the output signal S2 from the bandpass filter 2 comprises a ringing
signal which normally has a frequency from about 400 Hz to 500 Hz.
The signal channel 4 includes a symmetrical limiter 8 which brings
the signal S2 into a digital form. The output signal S6 from the
limiter 8 is supplied to a period measuring circuit 9. The period
measuring circuit 9 measures each period of the signal 6 by
measuring the time interval between successive positively rising
slopes of the signal S6. Each time the period measured is within a
range corresponding to the frequency range of the ringing signal,
i.e. within a range from 2 milliseconds to 2.5 milliseconds for the
above-mentioned frequency range between 400 and 500 Hz of the
ringing signal, the measuring circuit 9 furnishes a first digital
output signal S7. However, when a period of the signal S6 is
outside said range between 2 and 2.5 milliseconds, the measuring
circuit 9 furnishes a second digital output signal S7'. The first
and second output signals S7 and S7', respectively, appear at
different output terminals of the measuring circuit 9.
The counting input of a counter 10 is connected to the output of
the measuring circuit 9, furnishing the output signal S7, and thus
counts all successive periods of the signal S6 having periods
within the predetermined range. A reset input of the counter 10 is
connected to the other output of the measuring circuit 9 furnishing
the output signal S7'. Therefore, the counter 10 is reset to zero
whenever a period of the signal 6 is not within the predetermined
range.
The counter 10 has an output terminal at which an output signal S8
representing the momentary count Z of the counter 10 is available,
and to which a comparator 11 is connected. The comparator 11
compares the momentary count Z of the counter 10 with a
predetermined value N. The comparator 11 delivers a digital output
signal S9 when the count Z of the counter 10 equals the
predetermined value N. Thereby the output signal S9 appears
provided N periods of an audio signal S1 having a frequency between
400 and 500 Hz have arrived successively and without interruption
at the input terminal 1. The number N may be 50, for example.
However, if the audio signal S1 having a frequency between 400 and
500 Hz has been interrupted for any reason before reaching a time
length of N periods, the output signal S9 will not be generated in
view of the resetting to zero of counter 10 due to the
interruption. Therefore, a voice signal having a frequency portion
between 400 and 500 Hz cannot generate an output signal S9, because
it is highly improbable that any voice signal will show a narrowly
limited frequency component of relatively long duration. In other
words, the appearance of output signal S9 positively indicates that
the incoming signal S1 at terminal 1 is a ringing signal from the
telephone exchange.
In order to provide a useful signal representative of the presence
of a voice signal only, the digital output signal S5 from the delay
switch 7 and the digital output signal S9 from an inverter 12, to
which the output signal S9 from the comparator 11 is supplied, are
linked together by means of a logic element 13 which may be a
simple AND gate. Thus, the output terminal 14 of the present voice
detector, which is connected to the output of the AND gate 13,
provides a digital signal S10. The signal S10 has one of two levels
as, for example, a positive level when the signal S5 has a
corresponding level, and the signal S9 has the other binary level.
In other words, the signal S10 is supplied to the terminal 14
provided the input signal S1 supplied to the input terminal 1
simultaneously has the following characteristics:
It lies within a frequency range from 300 to 3600 Hz;
It has a minimum level;
It shows a minimum duration; and
It is not a periodical ringing signal with a frequency from 400 to
500 Hz.
Therefore, the output signal S10 indicates that the input signal S1
is a voice signal. The foregoing characteristics, i.e., bandpass
frequencies, minimum level, minimum duration and frequency range of
the ringing signal, may be adapted to the specific requirements, of
course.
As is readily apparent from FIG. 1 and the above description, the
circuit features described may be realized in a simple and
straight-forward manner in spite of the high discriminating
accuracy obtained. In the second signal channel 4, the limiter
preferably is a known Schmitt trigger circuit. Circuits 9 for
measuring the length of the periods of a periodical signal, in
particular of a digital periodical signal, are well known, and may
include a digital comparator which compares the digital result of
such measurement with a range of predetermined digital values to
deliver either signal S7 or S7' to the counter 10. Digital counters
10 are well known, too. The comparator 11 may form a part of the
counter 10 in that the counter 10, in a well-known manner,
generates an output signal S8 when it has reached its counting
capacity N.
As to the circuit elements 5, 6 and 7 of the first signal channel
3, a preferred embodiment will be described below in connection
with FIG. 2. As already mentioned, signal S5 in the channel 3 and
all signals from and including signal S6 in the channel 4 are
digital. Therefore, it may be of advantage to incorporate the
measuring circuit 9, the counter 10, the comparator 11, the
inverter 12 and the AND gate 13 as an additional function of a
microcomputer which forms part of a type of automatic telephone
answering apparatus as a central control unit. Such telephone
answering apparatus is known from the German Patent Application
Ser. No. 2,633,882, published on Feb. 2, 1978.
FIG. 2 represents a diagram of a preferred circuit combining the
functions of the threshold switch 5, the rectifier 6 and the delay
switch 7 as described above.
To obtain the threshold function, the two input terminals E1 and E2
of an operational amplifier 15 are maintained at fixed potentials
by means of a voltage divider including series resistors 16, 17 and
18 disposed between the positive supply voltage and the common
supply voltage or earth. The respective reference voltages are
supplied to the inputs E1 and E2 through isolating resistors 19 and
20, respectively. The signal S2 from the output of the bandpass
filter 2 (FIG. 1) is furnished to the second input E2 of the
operational amplifier through a capacitor 21. Therefore, a positive
voltage at the output A of the operational amplifier appears only,
if the positive amplitude of the signal S2 is higher than the
difference of potentials at the inputs E1 and E2.
With its base, a npn-transistor 22 is connected to the output A of
the operational amplifier 15, its emitter being connected to earth,
and its collector being connected to the positive supply voltage
through a load resistor 23. A series arrangement of a further
resistor 24 and a capacitor 25 is connected between the collector
of the transistor 22 and earth. An output line leads from the
capacitor 25 through an inverter 26 to the one input of the AND
gate 13 (FIG. 1). The resistor 23 has a relative high resistance of
47,000 ohms, for example. The resistor 24 has a much lower
resistance of 330 ohms, for example. In a practical embodiment, the
capacitor 25 had a capacitance of 2.2 microfarads. Therefore, with
non-conducting transistor 22, the time constant of the RC-circuit
including resistors 23, 24 and capacitor 25 is substantially larger
than that of the RC-circuit including resistor 24, capacitor 25 and
conducting collector-emitter path of transistor 22.
In the absence of any signal S2, transistor 22 is non-conducting,
and capacitor 25 is charged, therefore, to the level of the
positive supply voltage.
With a signal S2 having a positive amplitude of less than the
potential difference between inputs E1 and E2 of the operational
amplifier 15, there is no positive signal at its output A, the
transistor 22 remaining in its non-conducting state. In that manner
noise signal background on the telephone line is effectively
suppressed.
When the signal S2 exceeds the potential difference between the
inputs E1 and E2 in a positive direction, a positive output signal
drives the transistor 22 in its conducting state, the capacitor 25
discharging through the smaller resistor 24 and the transistor 22.
However, if the input signal S2 is of extremely short duration as
is the case with a noise peak, the capacitor 25 will not discharge
to an appreciable extent before the transistor 22 assumes again its
non-conducting state. In that way, short noise peaks have no
effect. Only if the signal S2 has a time duration beyond a
predetermined value allowing the capacitor 25 to discharge, a
negative signal will appear at the input of the inverter 26, and
give rise to a positive signal S5 at its output and, consequently,
at the one input of the AND gate 13.
As the signal S2 falls below the threshold determined by the
potential difference between the inputs E1 and E2 of the
operational amplifier 15, the transistor 22 assumes again its
non-conducting state. However, the capacitor 25 charges very slowly
through the larger resistor 23, and thus maintains a low potential
at the capacitor 25 for a longer time. Therefore, the positive
signal S5 at the output of the inverter 26 remains practically
constant for a time determined by the time constant of the resistor
23 and the capacitor 25. Thereafter, when the capacitor has again a
substantial positive charge, the inverter 26 charges the binary
value of its output signal S5.
In summary, the circuit of FIG. 2, despite its little number of
circuit components, favourable performs the function of the
threshold switch 5, the rectifier 6 and the delay switch 7
described in connection with FIG. 1. It generates an output signal
S5 only, if the input signal S2 has a minimum amplitude and a
minimum duration, and it further "stretches" the detected signal to
have a minimum duration after detection.
* * * * *